In general, a mass flow controller (MFC) controls and monitors the rate of fluid (i.e., a gas or vapor) flow in real time so that the flow rate of the mass of a gas passing though the device can be metered and controlled. Mass flow controllers (MFCs) are often used to control the flow of gases during a semiconductor manufacturing process wherein the flow of gases into a semiconductor tool, such as a vacuum chamber, must be carefully controlled in order to produce high yield semiconductor products. MFCs are usually designed and calibrated to control the flow rate of specific types of gas at particular ranges of flow rates. The devices control the rate of flow based on a given set point, usually predetermined by the user or an external device such as the semiconductor tool itself. The set point can be changed with each step of a process depending on the desired flow rate for each step. MFCs can be either analog or digital. They are typically designed to be used with pressure ranges of the inlet gases, with low pressure and high pressure MFCs being typically available. All MFCs have an inlet port, and outlet port, a mass flow meter including a mass flow sensor and a proportional control valve. A system controller is used as a part of a feedback control system that provides a control signal to the control valve as a function of a comparison of the flow rate as determined by the set point with the measured flow rate as sensed by the mass flow sensor. The feedback control system thus operates the valve so that the measured flow is maintained at the flow rate as determined by the set point.
Such control systems assume that the MFC remains in calibration within certain tolerances. In order to test whether a MFC is within the tolerances of calibration, the MFC is typically tested off line with such devices as mass flow verifiers. The latter are used to test the flow rates. While off line testing is very accurate, there is always a problem that a MFC can become out of calibration during the running of a process (in real time), and not be detected until the process is completed. Often this can result in lower yields of semiconductor product, and even a complete failure resulting in the loss of the entire product yield. This can be expensive, and is clearly undesirable. What is needed is a system and method for continually testing the accuracy of a MFC in real time while processes are being run.
Mass flow controllers include two types, thermal-based and pressure-based mass flow controllers. U.S. patent application Ser. No. 13/354,988 filed Jan. 20, 2012 in the name of Junhua Ding, entitled “System and Method of Monitoring Flow Through Mass Flow Controllers in Real Time”; and assigned the present assignee, describes a system and method for testing a thermal-based mass flow controller so that the accuracy of the mass flow controller can be verified without going off line.